The present invention relates to hydrogen silsesquioxane resin (H-resin) fractions derived from an extraction process using one or more fluids at, near or above their critical state. These fractions can comprise narrow molecular weight fractions with a dispersity less than about 3.0 or fractions useful for applying coatings on substrates. The invention also relates to a method of using these fractions for forming ceramic coatings on substrates.
Hydrogen silsesquioxane resin of the formula [HSiO.sub.3/2 ].sub.n is known in the art. For instance, Collins et al. in U.S. Pat. No. 3,615,272 describe a soluble resin produced by the hydrolysis of trichlorosilane, trimethoxsilane or triacetoxysilane in a hydrolysis medium formed by the reaction of concentrated sulfuric acid with an aromatic hydrocarbon. The resin obtained thereby has a large dispersity and comprises a wide range of molecular weight species including low molecular weight volatile components and high molecular weight gels.
Similarly, Bank et al. in U.S. patent application Ser. No. 07/401,726, now U.S. Pat. No. 5,010,159 issued on Apr. 23, 199l describe an analogous hydrolysis method which additionally involves neutralizing the resultant product with a base. As with Collins et al. supra, the resin produced by this method contains the same large dispersity and wide range of molecular weight species.
Methods of fractionating polymers are likewise known in the art. Such methods are generally liquid solution based and merely involve the addition of a non-solvent to a liquid solution of the polymer to precipitate out a desired fraction. Such processes are considered non-feasible for H-resin, however, since it is quite soluble and/or unstable in common solvents.
Supercritical fluid fractionation is also known in the art. For instance, Krukonis in Polymer News, vol. 11, pp. 7-16 (1985) describes the supercritical fluid fractionation of various silicon-containing polymers such as polydimethylsiloxanes and polycarbosilanes to produce narrow molecular weight fractions.
Yilgor et al. in Polymer Bulletin, 12, 491-506 (1984) also discuss the use of supercritical fluids in the separation. purification or fractionation of various chemical compositions to produce results which are difficult to obtain by conventional techniques. Included therein is a discussion of the use of this technique for fractionation and purification of functional siloxane oligomers.
The art also teaches that narrow molecular weight fractions of certain resins are superior for use in pressure sensitive adhesives (PSAs). For instance, European Patent Application, EP 255,226 teaches that MQ resins with a dispersity of about 2 or less when combined with polydimethylsiloxane gum provide PSAs with superior peel adhesion, quick stick, or shear.
Furthermore, the art also teaches that hydrogen silsesquioxane resin derived thin film ceramic coatings can be applied on various substrates including electronic devices and circuits. For instance, Haluska et al. in U.S. Pat. No. 4,756,977 disclose silica coatings produced by applying a solution of hydrogen silsesquioxane to a substrate followed by conversion of the resin to silica by heating to temperatures of 200.degree.-1000.degree. C. The ceramic coatings produced by this method are taught to have many desirable characteristics such as microhardness, moisture resistance, ion barrier, adhesion, ductility, tensile strength, low electrical conductance and thermal expansion coefficient matching that make such coatings especially valuable.
The present inventors have now discovered that hydrogen silsesquioxane resin can be fractionated into various novel molecular weight fractions and that these fractions are useful for the deposition of ceramic coatings on various substrates including electronic devices.